This invention relates to a novel waterless lithographic master of the planographic type and to a method for preparing said master.
In conventional lithography, an aqueous fountain solution is employed to prevent the ink from wetting the nonimaged areas of the planographic plate. It has recently been discovered that the requirement for a fountain solution can be obviated by employing a planographic plate having a silicone, i.e. organopolysiloxane, elastomeric layer. Because the silicone is not wetted by the printing ink, no fountain solution is required. While the use of silicone elastomers as a printing surface has obviated the requirement for a fountain solution, it has been found that finely divided particulate material commonly referred to in the trade as "toner", is not easily attached to the silicone. Thus, the abhesive or nonadhesive property of the silicone which renders it useful for rejecting lithographic inks, also causes it to reject other materials such as toner. Accordingly, it has been difficult to prepare a printing master in which the toner could be sufficiently attached to the silicone such that it would not become removed after a short run on a printing press.
In order to adhere a particulate imaging material to the abhesive silicone, it has been discovered that a copolymer can be employed comprising a major portion of siloxane blocks and a minor portion of organic thermoplastic blocks. This permits the master to be imaged with a particulate image material and the thermoplastic blocks softened and then hardened to bond the particulate imaging material thereto. Thus, the thermoplastic blocks permit the imaging material to be physically bonded thereto and the siloxane blocks provide an insoluble ink releasing background area so that no dampening or fountain solution is required. A difficulty encountered with the block copolymers, however, is that the background areas tend to ink slightly so as to impair the contrast. It is this problem to which this invention is directed.